Semiconductor elements such as transistors, IC, LSI, etc., are usually resin-encapsulated by plastic package. Such a resin encapsulation is performed using a thermosetting resin moldable material such as an epoxy resin composition, etc., by casting, compression molding, injection molding, transfer molding, etc., in particular, by transfer molding which is excellent in mass producibility and workability.
In such molding, a mold-releasing agent contained in the thermosetting resin moldable material oozes onto the surface of a cavity which is the molding space of a mold to show mold-releasing action. However, when such molding is repeated many times, inconveniences occur such that the releasing property of moldings is greatly reduced and the phenomena such as roughness, etc., occur on the surface of moldings to give no luster on the surface of moldings. The reasons are considered to be caused by that the mold-releasing agent containing in the above-described molding material is successively accumulated on the surface of the cavity of the mold by repeated molding and gradually oxidized and deteriorated to form a hard oxidized and deteriorated mold-releasing agent layer. That is, since the oxidized and deteriorated mold-releasing agent layer has neither smooth surface nor mold-releasing function, the above-described problems occur.
Accordingly, it has hitherto been performed the following method. That is, a melamin resin moldable material different from the above-described epoxy resin moldable material is placed in the cavity followed by molding to cure by heating, whereby the oxidized and deteriorated mold-releasing agent layer is attached to the hardened product in unitary body, and the hardened product is removed in its state from the cavity of the mold, whereby the surface of the cavity is cleaned.
However, in such a case, it is necessary to fill the melamin resin moldable material in many cavities of a mold and also withdraw the hardened product therefrom, the work being very troublesome as well as there is a problem that the working environment is greatly reduced by the odor emitted from the melamin resin moldable material at molding.
Thus, the inventors previously developed a sheet-form mold-cleaning composition comprising an uncured rubber composition containing therein a glycol ether (Japanese Patent Application No. 252536/86). The cleaning of mold by the sheet-form mold-releasing composition is performed by inserting the sheet-form composition between upper and lower molds, contacting the portion of the sheet corresponding to the surface of each cavity of the mold by the pressure at closing the mold, curing the composition by heating in its state to convert the whole uncured rubber composition into cured rubber, whereby the oxidized and deteriorated mold-releasing agent layer on the surface of each cavity is united with the cured rubber in unitary body, and then removing the sheet-form cured rubber from the mold to clean the surface of the cavities. By using such a mold-cleaning composition, the problem in the case of using the above-described thermosetting melamine resin moldable material does not occur. Also, the cavity surface of mold is cleaned to the initial mirror-surface state as the case of using the melamin resin moldable material.
However, in the case of cleaning the surface of cavities, etc., using the above-described mold-cleaning composition or the thermosetting melamin resin moldable material, the surface of mold, such as the cavity surface, etc., is cleaned to mirror-surface state and no mold-releasing agent exists on the surface of the mold. If the mold surface is completely cleaned, in the case of molding directly after cleaning, the molded article attached to the surface of the mold and cannot be smoothly released from the mold. This is because when the mold surface is completely cleaned, insufficient mold-releasing agent oozes from a moldable material in a single molding to sufficiently coat the surface of the mold.
Therefore, it has been proposed to spray a mold-releasing agent onto the surface of the mold after cleaning and has been practically performed. However, although mold-releasing property may be provided to the mold surface, in the case of employing the above-described method, problems occur such that a difference in concentration of mold-releasing agent occurs on the surface of the mold, as the case may be, the mold-releasing agent attaches to the surface of the molded article to reduce the appearance of the product, the mold-releasing agent also attaches to other portions of the mold than the molding surfaces at spraying to greatly reduce the molding condition, and further sucking of the sprayed mold-releasing agent injures the workers' health.
According, in practical operation, after cleaning a mold surface, an epoxy resin moldable material is molded to an extent of 5 shots (the molding is called "dummy molding") for sufficiently attaching the mold-releasing agent contained in the moldable material to the surface of the mold, and thereafter, conventional molding is carried out. However, it is very uneconomical to perform about 5 shots of such dummy molding, which causes the reduction of molding efficiency.